Fix code path in EvidenceQc when run_ploidy is false

[mwalker174]

Fixes a WDL error in the single sample pipeline when run_ploidy is false:

Failed to evaluate 'EvidenceQC.ploidy_plots' (reason 1 of 1): Evaluating select_first([CreateVariantCountPlots.ploidy_plots, Ploidy.ploidy_plots]) failed: select_first was called with 2 empty values. We needed at least one to be filled.

This is caused by the select_first() call to assign the optional ploidy_plots output.

Tested the EvidenceQc module in Terra with run_ploidy set to true and to false.

[kjaisingh]

LGTM - just remember to remove the branch from the dockstore sync.